The use of powdered metals, and particularly iron and its alloys, is known for forming permanent magnets, such as soft magnetic cores for transformers, inductors, AC and DC motors, generators, and relays. An advantage to using powdered metals is that forming operations, such as compression or injection molding and/or sintering techniques, can be used to form intricate molded part configurations, such as magnetic cores, without the need to perform additional machining and piercing operations. As a result, the formed part is often substantially ready for securement within its working environment as formed by the molding process.
Before sintering, the powdered metals are molded by techniques such as compression or injection molding. Molded magnetic cores for AC applications must have low core losses; therefore, the individual metal particles must be electrically insulated from each other. Numerous types of insulating materials, which also act as the binder required for molding, have been suggested by the prior art, including inorganic materials such as iron phosphate and alkali metal silicate. A list of the different organic polymeric materials suggested by the prior art is extensive and includes amber, phenol-aldehyde condensation products, varnishes formed from China-wood oil and phenol resin, resinous condensation products of urea or thiourea and their derivatives with formaldehyde, polymerized ethylene, butadiene, acrylic acid esters and their derivatives, copolymers of two or more of the above polymers as well as fluorine-type polymers, radical polymerizable monomers such as styrene, vinyl acetate, vinyl chloride, acrylonitrile, divinyl benzene, N-methylol acrylamide, silicones, polyimides, fluorocarbons and acrylics. In addition, it is known to coat a powdered metal with an inorganic undercoating and then provide an organic topcoat.
While the above materials generally provide adequate insulation and adhesion between metal particles upon molding, additional properties are often desirable of a coating material. One such property is the ability to provide lubrication during the molding operation so as to enhance the flowability and compressibility of the particles, and therefore enable the particles to attain maximum density and strength. This is true not only for molded articles, where as-molded strength and density are obviously required, but also for sintered applications, where the molded articles are further consolidated to attain even greater strength and density.
It is often preferable to sinter the magnetic core after the molding process, such as where the solid magnetic core is intended for DC applications. Sintering fuses the iron particles together to form a solid molded article and removes the polymer coating through volatilization. As a result, in addition to the abilities described above, the coating must be capable of being volatilized completely without leaving contaminants and voids within the core. The presence of contaminants or voids within the sintered article reduces the physical strength and properties of the sintered article.
In addition, a significant shortcoming associated with the use of the prior art coatings has been that the coatings will not burn off completely during sintering, thereby leaving a carbonaceous residue within the sintered article. This residue may actually diffuse into the underlying metal particle, causing some degree of deleterious carburization within the sintered article.
As disclosed in U.S. patent application Ser. No. 07/710,427, filed Jun. 7, 1991, assigned to the assignee of the present invention, polyetherimide, polyethersulfone and polyamideimide have been found to perform well as the coating material for powdered iron, so as to form insulated magnetic cores, particularly with respect to the ability to bind the iron particles together and resist thermal and chemical attack, and the ability to serve as a lubricant during the compression molding process. In addition, these polymers adhere well to the underlying metal particle. These polymers are applied to the iron particles using a fluidized bed process which is known in the art.
However, shortcomings associated with the teachings of U.S. Ser. No. 07/710,427 are two-fold. First, the above-described polymers may not compression mold suitably for certain applications due to insufficient lubricity. As a consequence, the magnetic cores may have unsuitably low densities which corresponds to lower magnetic permeability. In addition, the magnet cores tend to stick in the mold cavity, which further results in excessive tool wear and damaged parts. The current solutions to these shortcomings include blending lubricants with the powdered iron before molding and using mold release compounds, such as graphite, on the mold cavity prior to the molding cycle. However, the use of lubricants and mold release compounds may further reduce the density of the magnetic core and may introduce contaminants, such as carbon, into the material. During sintering, the presence of contaminants can cause voids or stress risers to be formed within the sintered article, or the contamination may diffuse into the underlying metal particle so as to detrimentally affect the alloys' properties. In addition, the above-described polymers tend not to volatilize completely upon sintering, therefore adding additional contaminants and/or voids to the sintered article.
Thus, it would be desirable to provide a coating for powdered metals which has the ability to improve lubrication during the molding process and to provide adhesion of the metal particles after molding, so as to attain maximum density and strength of the as-molded article. In addition, the coating should be readily and cleanly volatilized upon heating during a sintering process, so as not to leave contaminants or voids within the sintered article. Further, it would also be desirable if such a coating could be readily used for sintering of a variety of materials.